Factors Affecting Microbial Uranium Reduction: Implications for Bioremediation

Batch experiments were conducted to evaluate effects of environmental factors on U(VI) reduction by resting cells of Shewanella algae strain BrY. Rates of U(VI) reduction were examined at temperature 10-45°C, pH4-8, and dissolved-oxygen 0.0-8.6 mg/L. Rate of U(VI) reduction was fastest at 45°C (rate constant = 0.2/hr) and slowest at 10°C (rate constant = 0.02/hr). Holding temperature constant (20°C), rate of U(VI) reduction increased with increasing pH; rate constant increased from < 0.01/hr at pH4 to 0.08/hr at pH 8. In abiotic controls, however, increases in pH had little effect on U(VI) reduction, and the decrease in U(VI) was less than 15% of initial concentration, compared with > 80% in cell suspensions at the same pH. Oxygen content had little effect on U(VI) reduction when dissolved oxygen was <1.0 mg/L; at 8.6 mg/L concentration, however, reduction of U(VI) decreased by eight- to 10-fold. These results demonstrated that reduction of U(VI) was enhanced at temperature, pH, and redox conditions conducive to anaerobic growth or enzymatic viability of the metal-reducing bacterium. Transmission electron microscopy showed uranium mineral precipitation on cell membrane, suggesting that BrY effectively immobilized soluble uranium under the conditions examined.